At low frequencies, L1 looks like a short circuit, and C1 looks like an open circuit, so V(in) and V(out) are closely linked by R1. At high frequencies, L1 looks like an open circuit and C1 looks like a short circuit, so V(out) approaches zero. But in the middle range of frequencies, something interesting happens. In fact, for a narrow range of frequencies around 1MHz, this circuit actually produces a tremendous amount of voltage amplification!
TIME-DOMAIN SIMULATION:
We've configured V1
to put a square wave into the input, and if we plot the output, we can see a
tremendous amount of ringing in the output. Try changing R1 and seeing how it
affects the shape of the ringing! Also adjust L1 and or C1 to tune the exact
frequency of the resonance.
BODE PLOT:
Run the frequency
domain analysis to see a sharp peak of about +36dB around 1MHz. That means that
a signal around 1 MHz will get amplified by a factor of about 63x (10^(36/20) =
63)! That's a lot of gain, all from a passive circuit. That's why this kind of
inductor/capacitor combination is often found in radio equipment, as it can
select a narrow band of frequencies and amplify them without amplifying the
rest of the spectrum.
